THIS invention relates to particle separation according to the dielectric and electrophysical properties of the particles. In one application the invention relates to the separation of mineral particles according to their dielectric and electrophysical properties.
It is known to separate minerals using conventional electrostatic techniques in which particles are given electrostatic charges by induction or absorption of ions and electrons on the particle surface. These methods use corona discharge and other techniques. Examples of the known methods are described in, for instance, xe2x80x9cElectrostatic Separation of Granular Materialsxe2x80x9d (Bulletin 603, United States Department of the Interior, Bureau of Mines), Russian patent specification 2008976, U.S. Pat. No. 3,720,312 and UK patent specification 2130922. While such techniques are successful at least to some degree, they have a number of serious disadvantages.
Electrostatic techniques generally require relatively high voltages (typically 15 to 60 kV) and currents (typically of the order of 10 mA). This makes the separation process not only expensive to operate but also inherently dangerous. Another disadvantage is the fact that electrostatic techniques are sensitive to ambient atmospheric conditions such as humidity and temperature. Also, the productivity of conventional electrostatic methods is generally low. Generally such methods also require screening of the electrodes from dust and other surface contaminants which can degrade the operation of the separation apparatus. As a further disadvantage, conventional electrostatic separators tend to be large and complex.
It has also been proposed previously to separate mineral particles in accordance with their dielectric properties. Examples are described in Developments in Mineral Processing (Mineral Processing Vol.2, Part B, 1979, 1168-1194), Mineral Processing (3rd edition, E J Pryor, 588-594), Physical Basis of Electrical Separation (A. E Angelov et al, Moscow, Nedra 244-248, 1983), UK patent specification 2014061, Japanese patent specification 05126796A) and U.S. Pat. No. 4,473,452. The known methods have the disadvantage that ponderomotive forces required to cause spatial separation of particles with different dielectric constants are disguised by more powerful Coulomb and mirror forces arising from electrostatic interaction and so generally cannot be used in practice.
The present invention is based generally on the phenomenon known as electroadhesion and more particularly on the recognition of the importance of applying sharply non-homogeneous electrical fields to particles which are to be separated.
Electroadhesion is an effect by which particles can be held, by electrical attractive or repulsive forces, within a field set up between electrodes of various potentials. This effect can be attained most readily with electret materials, but is not restricted to such materials. An electret is a dielectric material which possesses persistent electrical polarisation. While the dipoles generally have a random orientation, under the influence of an applied electric field between oppositely charged electrodes, the individual dipoles align themselves and develop strong polarity which persists even after the initial field is removed. Typically the dipoles only revert back to a random orientation very slowly unless some exciting impulse is applied to them.
The application of a sharply non-homogeneous electrical field to the particles which are to be separated allows the generation of weak ponderomotive forces which are not dependent on polarity. The ponderomotive forces are generally much weaker than charge related Coulomb and mirror forces, accounting for only 1% to 3% of the total forces acting on the particles.
According to one aspect of the invention, there is provided a method of separating particles according to their dielectric and/or electrophysical properties, wherein particles which are to be separated are passed through a sharply non-homogenous electrical field, in a non-liquid medium, the electrical field having a gradient exceeding 108 V/m2 and a divergence exceeding 1011, such that particles with different dielectric and/or electrophysical properties are acted upon by different forces which separate them spatially, and spatially separated particles are collected in discrete fractions.
Preferably the sharply non-homogeneous electrical field is one having a gradient exceeding 4xc3x97109 V/m2 and a divergence exceeding 1012.
In one series of applications, relying on a combination of ponderomotive as well as Coulomb and mirror forces, the particles are passed through a sharply non-homogeneous electrical field set up between one or more DC electrodes and the sharp edge of a feeder. The particles are preferably passed through a combined, sharply non-homogeneous DC and AC electrical field. The particles may be discharged over a sharp feeder edge about which the combined field is set up. They may for instance be fed along a vibratory feeder to be discharged over a sharp edge thereof so as to fall under gravity through the combined, non-homogenous electrical field.
To ensure sharp non-homogeneity of the field and hence efficient separation of the particles, the radius of the feeder edge in these applications should be smaller than the particles. This dimension should be in the range 0,01 to 1 times the average particle diameter D, but is preferably in the range (0,01 to 0,5)D, most preferably in the range (0,01 to 0,1)D.
The feeder may be held at earth potential with a DC potential applied to a main space electrode situated adjacent the path of the particles as they are discharged from the edge of the feeder to set up a sharply non-homogeneous DC electrical field. A DC potential may also optionally be applied to a further electrode situated further than the main space electrode along the path of the particles discharged from the edge of the feeder. In this version, the particles are preferably conditioned prior to passage through the non-homogenous DC electrical field set up by the DC electrodes in an AC electrical field created by application of an AC potential to an electrode or electrodes situated above and/or below the feeder in the vicinity of the edge.
In another series of applications, in which particles are spatially separated from one another according to their dielectric properties, the particles are passed through a sharply non-homogeneous, high frequency AC electrical field. The AC electrical field may be set up by AC electrodes which are spaced apart from one another by insulating material in an electrode support structure. The electrodes may be, but are not necessarily, arranged parallel to one another in the electrode support structure and they are typically inclined to a direction in which the particles pass through the non-homogeneous electrical field.
The particles may be passed above or below the electrode support structure. This structure may be vibrated or the particles may be fluidised by a flow of air.
The method of the invention as summarised above is conveniently carried out in a gaseous medium, typically air.
According to a second aspect of the invention, there is provided an apparatus for separating mineral particles according to their dielectric and/or electrophysical properties, the apparatus comprising means for generating a sharply non-homogeneous electrical field having a gradient exceeding 108 V/m2 and a divergence exceeding 1011, feed means for feeding mineral particles which are to be separated through the electrical field such that particles with different dielectric and/or electrophysical properties are acted upon by different forces which separate them spatially, and spatially separated particles are collected in discrete fractions, and collection means for separately collecting the spatially separated particles.
Various further features of the method and apparatus summarised above are described below and set forth in the appended claims.
In one practical embodiment of the method and apparatus of the invention, particles of rutile (TiO2) can be separated from particles of zircon (ZrSiO4).